Septic tank design in the Philippines, from the rules of PD 856
Where a house cannot reach a public sewer, its waste water goes to a septic tank, and the tank is designed to the rules the Department of Health issued in 1995 under the Code on Sanitation, Presidential Decree 856. They set how the tank is sized, how deep the liquid stands, how far it sits from a well, what the health office approves, and when it is emptied. Every figure below was read from a scan of those rules and carries its section and page.
The septic tank rules, section by section
Each row gives the point, the rule in short, and where it is in the 1995 rules: the section, the page as printed, and the page of the scan.
Sewer or septic tank
| Point | The rule | Section and page |
|---|---|---|
| When a house needs its own system | no public sewer in an abutting street | Section 3.2.1(a), printed page 5, scan page 6: waste water piping is then provided with an individual sewage disposal system of approved type and design |
| When the public sewer counts as not available | more than 100 m away | Section 3.2.1(b), printed page 5, scan page 6: more than 100 meters from any proposed building on the lot |
| Where a sewer can be reached | connect, no septic tank | Section 3.2.1(c) and (d), printed page 5, scan page 6: no individual system on property accessible to a public sewer, and no Sanitary (Plumbing) Permit for one. Section 3.2.1(f), printed page 6, scan page 7: when a sewer becomes accessible, the individual system is abandoned and the house sewer connected |
| New subdivisions | public sewerage, as a rule | Section 3.2.3(g), printed page 8, scan page 8: no individual household system in a new subdivision unless a public collection system is impracticable and inadvisable there |
Permits and plans
| Point | The rule | Section and page |
|---|---|---|
| Before the building permit | health office checks the sanitary plans | Section 3.1.1, printed page 4, scan page 6: the Building Official refers every Sanitary (Plumbing) Permit application to the local health authority before the building permit issues |
| Before any work on a septic system | Sanitary (Plumbing) Permit | Section 3.2.2(a), printed page 6, scan page 7: the owner or an authorised agent files it with the local health authority before construction, installation, alteration or extension |
| What the health office may ask for | plot plan, signed plans, soil log | Section 3.2.2(c), printed page 6, scan page 7: a dimensioned plot plan with wells, water lines, structures and the system; plans and specifications signed and sealed by a Registered Sanitary Engineer or a Registered Master Plumber, as the case requires; and a log of soil and groundwater from test holes, with percolation test results |
| Before the tank is used | operational permit | Section 3.2.4(a), printed page 8, scan page 8: no one discharges effluent from an individual system unless the local health authority has approved an operational permit; 3.2.4(c): it may inspect and sample at any reasonable time |
Sizing the tank
| Point | The rule | Section and page |
|---|---|---|
| How many people it serves | 2 per room, or the actual number, whichever is greater | Section 4.1.1, printed page 9, scan page 8: for buildings with occupants, each room is counted as occupied by two persons, or the actual number served is used, whichever is the greater |
| Sewage flow for a small dwelling | 50 gallons a person a day | Table 1, Quantities of Sewage Flow, printed page 21, scan page 14: small dwellings and cottages 50; large dwellings with numerous fixtures 75 to 100; luxury residences and estates 100 to 150. Section 4.1.1 sizes the tank from this table, or from metered water use where available |
| Two tanks in series | the first holds at least half | Section 4.1.1, printed page 9, scan page 8: tanks in series may make up the capacity provided the first furnishes at least half of it |
| Two or more compartments | first holds 1/2 to 2/3 of the volume | Section 4.1.3(a), printed page 10, scan page 9: the first compartment has from one half to two thirds of the total volume; the tank may be any shape that meets capacity, inlet, outlet and depth, and is generally rectangular |
Inside the tank
| Point | The rule | Section and page |
|---|---|---|
| Liquid depth | at least 1.20 m | Section 4.1.3(b), printed page 10, scan page 9: a liquid drawing depth not less than 1.20 meters (4 feet) |
| Air space above the liquid | at least 20 cm | Section 4.1.3(c), printed page 10, scan page 9: from the liquid level to the inside top of the tank |
| Inlet above outlet | at least 5 cm | Section 4.1.2(e), printed page 9, scan page 8: the invert of the inlet pipe not less than 5 cm above the invert of the outlet pipe; 4.1.2(a): the inlet invert not less than 5 cm above the liquid level |
| Inlet baffle or sanitary tee | at least 15 cm below the liquid | Section 4.1.2(b), printed page 9, scan page 8: vented, turning the incoming sewage downward, but no deeper than the outlet device |
| Manholes | one per compartment, at least 0.36 m² | Section 4.1.4, printed page 10, scan page 9: or an equivalent removable cover slab; under concrete or block paving, the openings are extended to grade |
| Rain and ground water | kept out | Section 4.1.5(d), printed page 11, scan page 9: roof drains, foundation drains, area drains and cistern overflows may not enter the tank or any part of the system |
| The tank itself | watertight, level, approved plans | Section 4.1.5(a) and (b), printed pages 10 and 11, scan page 9: plans showing dimensions, reinforcement and structural calculations go to the local health authority; the tank is watertight, durable, designed for earth loads and installed level on a solid bed. 4.1.5(c): approved prefabricated tanks may be used, filled with water after installation so they do not float |
Where the tank goes
| Point | The rule | Section and page |
|---|---|---|
| From a well, spring or cistern | at least 25 m | Section 4.1.6(a), printed page 11, scan page 9: from any well, spring, cistern or other source of drinking water supply |
| From a water service line | at least 1.5 m | Section 4.1.6(a), printed page 11, scan page 9 |
| From a water main | at least 3.0 m | Section 4.1.6(a), printed page 11, scan page 9 |
| Under the house | not allowed | Section 4.1.6(c), printed page 11, scan page 9: septic tanks shall not be located under the building |
| Access for the desludging truck | required | Section 4.1.6(b), printed page 11, scan page 9: located so desludging equipment can reach the manholes conveniently |
Looking after it
| Point | The rule | Section and page |
|---|---|---|
| Inspection | at least once a year | Section 4.1.7(b), printed page 11, scan page 9 |
| When to desludge | scum near the outlet, or capacity down by half | Section 4.1.7(b), printed page 11, scan page 9: when the bottom of the scum mat is within "7.50 (3 inches)" of the bottom of the outlet device, as the scan prints it with no unit, which is about 7.5 cm; or when sludge and scum have reduced the liquid capacity by 50% |
| After cleaning | no washing, leave a little sludge | Section 4.1.7(c), printed page 11, scan page 9: tanks are not washed or disinfected, and a small residual of sludge is left for seeding |
| The sludge taken out | never to a field, ditch or water | Section 4.1.7(d), printed page 11, scan page 9: buried or disposed of by an approved method |
Where the effluent goes
| Point | The rule | Section and page |
|---|---|---|
| Onto the ground, a street or a canal | prohibited | Section 3.2.3(a), printed page 7, scan page 7: untreated sewage and septic tank effluent may not be discharged onto the ground surface, a street, a storm sewer, a drain ditch, adjoining property or any watercourse or body of water |
| Into an old well or rock openings | prohibited | Section 3.2.3(b), printed page 7, scan page 7 |
| Leaching field: groundwater or rock | at least 1.20 m down | Section 3.2.3(c), printed page 7, scan page 7: no leaching field, bed or well where normal groundwater or rock is less than 1.20 meters deep. Section 4.2.1(e)(iii), printed page 13, scan page 10: 1.2 m below the bottom of the device |
| Leaching field: the soil | percolation test | Section 3.2.3(d) to (f), printed page 7, scan page 7: a percolation test by a registered civil or sanitary engineer; no tile field or bed where water falls less than 2.54 cm in 60 minutes in the test holes, and no seepage pit or leaching well where it falls less than 2.54 cm in 30 minutes. 4.2.1(e)(i): none in swampy ground or where ponding or flooding is likely |
| Leaching field: distances | 25 m from a well, 3 m from the house, 1.5 m from a lot line | Section 4.2.3, printed page 15, scan page 11: not less than 25 meters from any well, spring, cistern or drinking water source; 3 m from an occupied building; 1.5 m from any lot line |
The pipe from the house
| Point | The rule | Section and page |
|---|---|---|
| House sewer size | at least 100 mm | Section 4.3.1, printed page 15, scan page 11: sized by the fixture units it drains, never under 100 mm (4 in.) in diameter |
| House sewer slope | 1:50, or 1:100 where that cannot be had | Section 4.3.3(a), printed page 15, scan page 11: a uniform slope of not less than 1:50 toward the point of disposal; where depth or the building makes that impracticable, pipe of 100 mm or more may run at 1:100. The scan prints these as "(0.02%)" and "(0.01%)"; as ratios they are 2 and 1 percent, and the page gives the ratios |
| Near a well | pressure tested | Section 4.3.3(b), printed page 15, scan page 11: a house sewer within 25 meters of a well, spring or drinking water source must pass a ten foot head of water test for fifteen minutes |
If the effluent goes to a leaching field
A tank settles and digests the solids; the liquid that leaves it still has to go somewhere lawful. Where it soaks into the ground through a leaching field or bed, the area of trench bottom depends on how fast the soil takes water in a percolation test. The rules give this table "for residences", in Section 4.2.1(d), printed page 12, scan page 10. A trench from 0.45 m to 0.90 m wide is a leaching tile field, and wider than 0.90 m it is a leaching bed (4.2.1(a) and (b)).
| Time for water to fall 2.54 cm in the test | Soil | Trench bottom required |
|---|---|---|
| 1 minute or less | good | 6.5 m² |
| 5 minutes | good | 11.61 m² |
| 10 minutes | good | 15.33 m² |
| 15 minutes | fair | 17.65 m² |
| 30 minutes | fair | 23.22 m² |
| 45 minutes | poor | 27.87 m² |
| 60 minutes | poor | 30.66 m² |
Slower than 60 minutes, no leaching field or bed may be built at all (Section 4.2.1(e)(ii)). Nor where the ground floods or ponds, or where groundwater or rock is less than 1.2 m below the bottom of the device. For a house on a lot that floods, the elevated house design guide covers raising the floor.
What it means when you build
- Find out first whether a sewer can be reached. If a public sewer runs within 100 m of the house, the rules expect a connection, and no permit issues for a septic system on that lot. The Clean Water Act guide covers the connection duty that RA 9275 later added.
- A well on the lot decides where the tank can go. The tank and any leaching field sit at least 25 m from a well, spring or cistern. On a small lot with its own well, that single figure can decide the site plan, so look at it before the house is placed. Our consultation for future home buyers reads a lot for this before you commit.
- Never under the house, always reachable. The tank may not sit under the building, and the desludging truck must reach its manholes. Under a paved garage, the manholes are brought up to the paving.
- The sanitary plans come before the building permit. The Building Official sends the Sanitary (Plumbing) Permit application to the local health office before the building permit issues, so the tank is on the drawings from the start. The building permit guide lists the rest of the set, and the house plan guide shows where the sanitary sheet sits in it.
- Roof water stays out of the tank. Roof, foundation and area drains may not enter it; they go to the storm drainage.
What this page does not cover
- Later amendments. This page reads the 1995 rules as issued. If the Department of Health has amended Chapter XVII's rules since, that amendment is not reflected here and could not be checked for this page. Ask the local health office which version it applies.
- The plumbing code. Pipe sizing by fixture units and the rest of the plumbing inside the house are set by the plumbing code, which is not read here.
- Penalties. Section 11 makes a violation a misdemeanour punishable by a fine, imprisonment of up to six months, or both; the amount is not printed here.
- Privies, public sewerage and establishments. The rules also cover sanitary privies, public sewer systems, and buildings other than houses; they are left out.
Source: Implementing Rules and Regulations of Chapter XVII, "Sewage Collection and Disposal, Excreta Disposal and Drainage", of the Code on Sanitation of the Philippines (P.D. 856), Department of Health, approved 21 December 1995 and published 22 February 1996, as scanned by IRC WASH (document 822-95-14761). The Code itself is on the Lawphil Project.